There is known a substrate treating device used in a wet etching process for an electronic component such as a semiconductor device or a liquid crystal display (for example, see Jpn. Pat. Appln. KOKAI Publication No. 11-26425). Concerning the substrate treating device, as a method of removing a strained layer formed on a substrate surface or an impurity such as a heavy metal existing on a substrate surface in the manufacturing process of a substrate such as a semiconductor wafer, a method of etching the substrate surface by a drug solution is known.
As the etching method, for example, a method of performing etching by feeding an etchant prepared by mixing hydrofluoric acid (HF), nitric acid (HNO3), and pure water at a predetermined mixing ratio to a semiconductor wafer is used. It is known that nitrous acid (HNO2) in the etchant contributes to etching. For this reason, an Si substrate (for example, semiconductor wafer) is dissolved in a solution containing hydrofluoric acid and nitric acid, thereby generating nitrous acid and thus generating an etchant.
The above-described substrate treating device has the following problem. That is, if the nitrous acid concentration in the etchant is low, the etching process time of the semiconductor wafer becomes long. Hence, the Si substrate needs to be dissolved in a sufficient amount. However, the Si substrate takes time to dissolve, and productivity lowers.